The radiographic and photographic advantages of tabular grain emulsions exhibiting high tabularity were first generally appreciated in the early 1980's.
Tabular grain emulsions are those emulsions in which tabular grains account for greater than 50 percent of total grain projected area. Tabular grains are those that contain two parallel major faces that are clearly larger than any remaining face.
Tabular grain emulsions recognized to be advantageous were initially characterized in terms of their average aspect ratios, where aspect ratio is defined by the following relationship: (I) EQU ECD.div.t=AR
where
AR represent aspect ratio; PA1 ECD represents tabular grain equivalent circular diameter; and PA1 t represents tabular grain thickness. PA1 AR.sub.av. is average aspect ratio; PA1 T is tabularity; and PA1 ECD.sub.av. and t.sub.av. are as defined above, but in this instance both are measured in micrometers (.mu.m). PA1 ECD.sub.av. is tabular grain average equivalent circular diameter in micrometers (.mu.m) and PA1 t.sub.av. is tabular grain average thickness in .mu.m.
The average aspect ratio (AR.sub.av.) of a tabular grain emulsion can be determined as the average of the tabular grain aspect ratios or, more easily, as the quotient of the average ECD (ECD.sub.av.) and average t (t.sub.av.) of the tabular grains. High aspect ratio tabular grain emulsions are those in which AR.sub.av. is &gt;8. Intermediate aspect ratio tabular grain emulsions are those in which AR.sub.av. is in the range of from 5 to 8.
An alternative characterization of tabular grain emulsions is in terms of tabularity (T). High tabularity tabular grain emulsions are those that satisfy the relationship: (II) EQU T=&gt;25=ECD.sub.av. .div.t.sub.av..sup.2 =AR.sub.av. /t.sub.av.
where
Kofron et al U.S. Pat. No. 4,439,520 disclosed the first chemically and spectrally sensitized high aspect ratio and high tabularity tabular grain emulsions. Wilgus et al U.S. Pat. No. 4,434,226 reported the preparation of high aspect ratio and high tabularity silver iodobromide tabular grain emulsions with the iodide substantially uniformly distributed within the grains. (All references to mixed halide grains identify halide in an ascending order of halide concentrations.) Solberg et al U.S. Pat. No. 4,433,048 reported high aspect ratio and high tabularity silver iodobromide containing varied iodide concentrations within the tabular grains.
A variety of photographic advantages were observed, including the following having direct applicability to radiography: improved-speed granularity relationships; a capability of more rapid processing; higher contrast for a given level of grain size dispersity; and less image variance as a function of processing time and/or temperature variances.
Concurrently Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426 reported reduced crossover in double-coated (Dupltized.TM.) radiographic elements containing spectrally sensitized high tabularity tabular grain emulsions.
Also concurrently, Dickerson U.S. Pat. No. 4,414,304 reported thin (t.ltoreq.0.2 .mu.m) tabular grain emulsions to exhibit both increased covering power and reduced variance in covering power as a function of the degree of forehardening. This addressed a long standing need in radiography, since the practice in the art prior to this discovery was to forego full forehardening of radiographic elements to avoid excessive loss of covering power, necessitating the completion of hardening during processing after imagewise exposure.
Because of their significant and multiple performance advantages, high tabularity tabular grain emulsions of the silver bromide and iodobromide compositions conventionally employed in radiography were promptly incorporated into commercial radiographic elements.
While high aspect ratio and high tabularity silver bromide and iodobromide emulsions have advanced the state of the art in almost every grain related parameter of significance in silver halide radiography, one area of concern has been the susceptibility of these emulsions to vary their imaging response as a function of the application of localized pressure to the grains. These are observed as localized variations of density (hereinafter referred to as pressure marks) superimposed upon the image information. For example, medical radiographic films are generally coated in large film sizes (e.g., up to 40 cm.times.40 cm) to obtain full size images of large body portions, such as the thoracic (chest) cavity. These radiographic films require increased care during manufacturing operations, such as cutting to size and packaging, to avoid pressure marking. Unfortunately, the user, the X-ray lab technician, does not always appreciate or implement increased care in handling. Pressure marks can be generated by manual or equipment handling of the film. Pressure marks can be produced by kinking caused by holding a large film sheet by one edge or corner. Kink marks appear as crescent shape pressure marks. Automatic film loaders and exposure devices have been observed to produce pressure marks attributable to misaligned guide pins and rollers applying excessive pressure to the film. Pressure marks are objectionable in all radiographic imaging applications, and are particularly objectionable in medical diagnostic applications, since pressure marks can be mistaken for or obscure pathology features in the radiographic image.